Masks used in semiconductor fabrication are manufactured by electron-beam lithography. Manufacture of such masks is disclosed in U.S. Pat. No. 4,652,762 and described next by referring to FIG. 1(a) to FIG. 1(f).
First, a glass substrate 1 as shown in FIG. 1(a) is prepared. A thin film 2 which does not transmit light is formed on the substrate 1 by vacuum deposition, as shown in FIG. 1(b). The film 2 is made from chromium (Cr), for example. As the film 2 is deposited, the internal stress and thermal stress in the thin film deform the substrate 1. Then, a resist 3 is applied to the thin film 2, as shown in FIG. 1(c). Thereafter, desired portions of the resist 3 are irradiated with an electron beam EB, as shown in FIG. 1(d). In this example, the parallel lines written by the beam EB are spaced apart from each other by a distance of a. Subsequently, the resist 3 is developed, as shown in FIG. 1(e). If the resist 3 is positive-acting, only the irradiated portions of the resist are removed by the development. After the completion of the development of the resist, only the irradiated portions of the thin film are etched away, as shown in FIG. 1(f). In this condition, most of the substrate 1 is still covered with the thin film 2. Therefore, the deformation of the glass substrate 1 makes little difference with the deformation of the substrate in the condition shown in FIG. 1(b). After the thin film has been removed, belt-like portions are created by the etching. Let b be the distance by which the etched portions are spaced apart from each other. The distance b is different from the distance a but their difference is comparatively small.
Where a negative resist is used, the lithography steps illustrated in FIGS. 1(a)-(d) are carried out also in the case of the positive resist. When the resist is developed, it is removed except for the irradiated portions, as shown in FIG. 2(a). After developing this resist, an etching process is carried out. As a result, only the thin film under the remaining resist portions is left behind; the thin film has been etched away in all the other locations, as shown in FIG. 2(b).
Where a negative resist is employed in this way, only those portions of the thin film which were irradiated with the electron beams are left behind. In this case, the thin film is removed from most of the substrate 1. The substrate that was deformed because of the internal stress in the thin film and other stress then returns to its original condition shown in FIG. 2(b). Consequently, the interval c between the remaining portions of the thin film differs considerably from the interval a between the lines drawn by the electron beam. The result is that the pattern is created with lower accuracy.